Sexual Selection. Male and female. Lukas Schärer. Evolutionary Biology Zoological Institute University of Basel

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1 Sexual Selection Lukas Schärer! Evolutionary Biology Zoological Institute University of Basel Advanced-level Evolutionary Biology Male and female 2

2 Summary: Sexual Selection defining sexual selection mating systems measuring sexual selection models of sexual selection male-male competition and female choice sperm competition and cryptic female choice 3 Defining sexual selection Darwin s definitions 1st edition of On the Origin of Species by Means of Natural Selection (1859) (sexual selection) depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring 6th edition of On the Origin of Species by Means of Natural Selection (1872) (sexual) selection depends, not on a struggle for existence in relation to other organic beings or to external conditions, but on a struggle between the individuals of one sex, generally the males, for the possession of the other sex. The result is not death to the unsuccessful competitor, but few or no offspring 1st edition of The Descent of Man and Selection in Relation to Sex (1871) (sexual selection) depends on the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction 2nd edition of The Descent of Man and Selection in Relation to Sex (1874) (sexual selection) depends on the advantage which certain individuals have over others of the same sex and species solely in respect of reproduction 4

3 Defining sexual selection sexual selection is a component of natural selection note also that mating success does not have to be strongly correlated to fertilisation success 5 from Arnqvist & Rowe 2005 Defining sexual selection Bateman s principle (like many principles is only a rule of thumb) in a female, reproduction tends to be limited by the number (and quality) of eggs she produces, which is often limited by the amount of resources she has available in a male, reproduction tends to be limited by the number (and quality) of eggs he fertilises, which is often limited by the number of females he can mate with 6 from Bateman 1948

4 Defining sexual selection Darwin (1874) defines two types of sexual selection! the wording in the 1871 edition is only slightly different intrasexual selection (generally seen as male-male competition) The sexual struggle is of two kinds; in the one it is between individuals of the same sex, generally the males, in order to drive away or kill their rivals, the females remaining passive intersexual selection (generally seen as female choice) whilst in the other the struggle is likewise between the individuals of the same sex, in order to excite or charm those of the opposite sex, generally the females, which no longer remain passive, but select the more agreeable partners 7 Defining sexual selection female choice (female preference, mating bias) any female trait (e.g., behaviour, morphology or physiology) that influences the reproductive success of certain males based on their phenotype does not need to be a conscious decision does not even have to increase female fitness the same caveats may apply to male choice, and recent studies suggest there is probably often mutual mate choice 8

5 Mating systems the mating system concept aims to broadly classify how males and females interact sexually Cygnus buccinator! monogamy each sex has a single mate for life polygyny females mate with a single male, but males mate with more than one female (lekking or haremic systems) Mirounga angustirostris polyandry males mate with a single female, but females mate with more than one male (sex-role reversal) Phalaropus lobatus polygamy both sexes have several partners and approximately equal variation in mate numbers Thalassoma bifasciatum 9 Mating systems the mating system in a bushcricket is dependent on the feeding status 10 from Gwynne & Simmons 1990

6 Mating systems the social mating system does not have to be to equal the genetic mating system red-winged blackbird (Agelaius phoeniceus) 11 Measuring sexual selection sexual selection needs to be measured quantitatively Variance 12 from Arnqvist & Rowe 2005

7 Measuring sexual selection variance in reproductive success in red deer (Cervus elaphus) what are the traits that determine where an individual lies in this distribution? are those traits expressed in males, in females, or in both sexes? how do such traits originate and evolve? % of females % of males 13 Models of sexual selection Fisherian runaway process (think sexy sons ) females prefer a random male trait, and the offspring inherit both the trait and the preference for the trait what is the origin of the trait and the preference? the resulting genetic correlation can lead to a runaway process, which is eventually stopped if the costs get too high, due to e.g. direct viability selection on the male trait viability selection on correlated female trait (leads to sex-limited expression!) costs of female choice 14 from Stearns & Hoekstra 2005

8 Models of sexual selection handicaps, indicators and good genes whitefish (Coregonus sp.) females prefer males with traits that indicate good genetic quality, and the offspring inherit these good genes and the preference for them but genetic quality is expected to be already under directional selection, and so it is unclear what maintains the variation in male quality within the population 15 from Wedekind et al Models of sexual selection direct effects females prefer males that provide material benefits (e.g., a nuptial gift, a good territory or sufficient sperm) these benefits may either be measured directly or based on a trait that is correlated with them direct effects usually do not lead to extreme traits but preferences for direct effects can be exploited the dance fly (Rhamphomyia sulcata) 16 from LeBas & Hockham 2005

9 Models of sexual selection sensory exploitation males exploit an existing sensory bias in females (e.g., a preference for red dots, because the females eat red berries) this can be the starting point of a Fisherian runaway process the females cannot easily evolve resistance against this kind of exploitation (e.g. because they still need to find red berries) note that the berry example is only a thought experiment! 17 from Basolo 2002 Male-male competition and female choice intrasexual selection (male-male competiton) contest competition rivals physically compete for the mating partner scramble competition rivals compete via agility or speed endurance rivalry rivals compete through endurance intersexual selection female preference for condition and parasite resistance females choose a good quality male female preference for an exaggerated trait females chose based on an extended phenotype female preference for complementarity females choose the right partner given their own genetic composition (i.e. there is no overall best male!!) 18

10 Intrasexual selection contest competition in the dung beetle genus Onthophagus males fight with their horns and females mate with the winner 19 Intrasexual selection horn size and shape varies greatly between Onthophagus species back of the head middle of the head front of the head center of the pronotum sides of the pronotum 20 from Emlen et al. 2005

11 21 from Emlen et al from Emlen et al. 2005

12 Intersexual selection female preference for a complementary MHC genotype the MHC is important for antigen presentation in the immune system 23 Intersexual selection women ranked the pleasantness and intensity of the smell of t-shirts worn by different men the smell of t-shirts worn by MHC dissimilar men was considered to be more pleasant but the preference was inverted when the women were taking the contraceptive pill modern contraceptive pills use different hormones and/or formulations 24 from Wedekind et al. 1995

13 Male-male competition and female choice sexual selection occurs at both the pre- and postcopulatory level Darwin may not have realised that the second level can be important and assumed that the number of matings is a good proxy for reproductive success pre- and post-copulatory sexual selection can be linked in the scorpion fly (Hylobittacus apicalis) bigger nuptial gifts lead to longer copulations and greater sperm transfer, which is advantageous under conditions of sperm competition 25 from Krebs and Davies 1993 after Thornhill 1976 Sperm competition and cryptic female choice definitions of sperm competition by Parker competition within a single female between the sperm of two or more males for the fertilisation of the ova Parker (1970) in: Sperm competition and its evolutionary consequences in the insects. Biological Reviews 45: competition between the sperm of two or more males for the fertilisation of a given set of ova Parker (1998) in: Birkhead & Møller (eds.). Sperm Competition and Sexual Selection. San Diego, CA, USA: Academic Press.! do not try to think of fighting sperm, but rather of competition between different ejaculates 26

14 Sperm competition and cryptic female choice expenditure on gaining matings or ejaculates 27 from Parker 1998 Sperm competition and cryptic female choice the evolution of testis size in primates 28 from Dixson & Anderson 2002

15 Sperm competition and cryptic female choice definitions of cryptic female choice a female-controlled process or structure that selectively favours paternity by conspecific males with a particular trait over that of others that lack the trait when the female has copulated with both types. Eberhard (1996) in: Female Control: Sexual Selection by Cryptic Female Choice. Princeton University Press non-random paternity biases resulting from female morphology, physiology or behaviour that occur after coupling. Pitnick & Brown (2000) in: Criteria for demonstrating female sperm choice. Evolution 54: ! and as previously explained for normal female choice, this does not have to be a conscious choice 29 Sperm competition and cryptic female choice sperm rejection by domestic chicken chicken reject sperm by subdominant males 30 from Pizzari & Birkhead 2000

16 Summary: Sexual Selection defining sexual selection mating systems measuring sexual selection models of sexual selection male-male competition and female choice sperm competition and cryptic female choice 31 Literature Mandatory Reading Pages in Chapter 1 of Birkhead (2000). Promiscuity: an Evolutionary History of Sperm Competition. Harvard University Press Suggested Reading Chapter 11 on Sexual Selection in Stearns & Hoekstra (2005). Evolution: An Introduction. 2nd Edition. Oxford University Press Chapter 20 on Sexual Selection by Jennions and Kokko (2010) in Evolutionary Behavioral Ecology (Westneat and Fox, eds.) Books Birkhead (2000). Promiscuity: an Evolutionary History of Sperm Competition. Harvard University Press 32